In recent years, along with the increase of the amount of information, an optical recording medium having a higher capacity is demanded. As one method for obtaining a high capacity, a multilayer technique may be mentioned wherein a multilayer type optical recording medium is formed by laminating plural aggregates comprising one recording layer, a reflective layer required for recoding or reproducing information on the recoding layer, a protective layer, etc. (hereinafter, the aggregate is referred to as “recording/reproducing functional layer”) with a distance at a level of optically dividing the aggregates, whereby the capacity is remarkably increased.
In general, since it is not necessary to increase the transmittance of a recording/reproducing functional layer outermost from the incidence side of laser light (hereinafter referred to as “outermost recording/reproducing functional layer”) for a multilayer type optical recording medium, it is possible to design an optical recording medium similar to one layer type optical recording medium.
On the other hand, since recording/reproducing functional layers (hereinafter, referred to as “transmission recording/reproducing functional layer”) other than the outermost recording/reproducing functional layer are required to have a predetermined transmittance, only a part of incident light is used for recording or reproducing information. Further, only a part of incident light is also used for recording or reproducing information on recording/reproducing functional layers other than the innermost recording/reproducing functional layer from the laser incidence side due to the existence of a recording/reproducing functional layer on its front side. That is why, as compared to one layer type optical recording media, information intensity obtained from each recording/reproducing functional layer in a multilayer type optical recording media is remarkably weak. Thus, a reproducing system for multilayer type optical recording media is required to read a weak information intensity, and many such reproducing techniques have been developed in recent years.
In general, a transmission recording/reproducing functional layer comprises a protective layer, a recording layer, a protective layer, a reflective layer and a protective layer in this order from the incidence side of laser light. It is considered that in order to improve the transmittance of the transmission recording/reproducing functional layer, a material having a high transmittance is used as a material for each layer constituting the transmission recording/reproducing functional layer, or the thickness of each layer constituting the transmission recording/reproducing functional layer is made to be thin. For example, it is considered that the recording layer or the reflective layer is made to be thin, however, since such a layer remarkably contributes to signal intensity thermally and optically, the thinner the recording layer or the reflective layer is, the more the recording/reproducing properties tend to deteriorate. Therefore, a thinner layer is preferred from the viewpoint of the transmittance, and it is not always possible to select a layer structure so as to form recording marks appropriately by giving priority to optimization of the recording property.
Therefore, in the case of the transmission recording/reproducing functional layer, the range of controlling the thickness of the reflective layer and the recording layer tends to be narrow inevitably. More accurately, the recording/reproducing properties including the reflectivity and the transmittance of the recording/reproducing functional layer are decided by the combination with a protective layer formed above or below the reflective layer or the recording layer, however, the range of controlling the film thickness or the like tends to be extremely narrow, as compared to a case where it is not necessary to lower the transmittance.
Further, particularly in a case of rewritable phase change type optical recording media, a reflective layer has a role to transmit heat from a phase change recording layer which has absorbed laser light and to form proper amorphous marks. In experiments, by providing a thick reflective layer having a large heat conductivity, an optical recording medium on which recoding and deleting information can be easily carried out can be designed. However, it is practically very difficult to design an optical recording medium having a thin reflective layer in order to produce multilayer type optical recording media.
Nonetheless, in a case of a recording/reproducing functional layer at the laser light incidence side in a rewritable phase change type optical recording medium having two recording/reproducing functional layers, if a recording linear rate is not high, signal properties required for commercialization can be obtained by using an Ag alloy at a level of 10 nm as a reflective layer, and carefully designing a film thickness of a recording layer, a protective layer, etc. However, it is difficult to apply such a technique to multilayer type optical recording media having more than two layers at the present stage, and it is also not expected to have the recording linear rate improved.
Next, a protective layer between a recording layer and a reflective layer is a layer for releasing heat from a recording layer heated by absorbing light energy to the reflective layer. Depending on the thickness of the protective layer, the heat release property from the recording layer can be controlled. However, since the range of controlling the thickness of the recording layer and the reflective layer is narrow, the thickness of the protective layer to obtain an appropriate heat release property is almost decided. Therefore, the range of controlling the thickness of the protective layer is small, and in general, the thickness of the protective layer is thinner than the two other protective layers.
Thus, the transmittance of the recording/reproducing functional layer is controlled by two outer protective layers in most cases.
The transmittance of the two outer protective layers is controlled by controlling the reflectivity by utilizing an interference effect of a material (dielectric) used for the two outer protective layers. Usually, the refractive index of transparent resins and substrates is about 1.5 in many cases. However, by making the recording/reproducing functional layer have a higher refractive index than the transparent resin and the substrate which sandwich the recording/reproducing functional layer, the reflectivity can be freely controlled.
Further, since the thickness of the two outer protective layers is thick at a certain level in many cases, the two outer protective layers preferably have a high transparency, particularly preferably have a high transparency at the wavelength of laser light used for recording and reproducing (hereinafter sometimes referred to as “recording/reproducing laser wavelength”). In the optical recording medium, for example, recording/reproducing laser having a wavelength of at least 450 nm is used in many cases, and materials which are preferably used at the above wavelength are restricted.
Further, in a case where a protective layer having a thickness at a certain level is produced by sputtering or the like, a material which has a high film formation rate is preferred.
For the reason stated above, the material for the protective layer is required to satisfy properties such that the refractive index is high, the absorption is small, the film formation rate is high, etc., and as a preferred material, a material containing ZnS as a main component is preferably used. For example, (ZnS)80(SiO2)20 or the like is widely used as a material for phase change optical disks (Patent Document 1).